Sources of high-power CW laser radiation at visible wavelengths, for example, green laser-radiation having a wavelength of 532 nanometers (nm) have become of great interest for industrial applications, for example re-crystallization of amorphous silicon layers in display panels, or welding of highly reflective metals such as copper and gold. In the welding application, the “green” wavelength promotes absorption of the beam in the material and leads to void-free welded seams. A power of several hundreds of Watts (W) of CW radiation is required for these applications.
Several different laser arrangements are presently employed for providing high-power visible wavelength CW radiation. Efficiency, reliability and cost are important factors in selecting which arrangement to use for any particular application.
CW external-cavity surface-emitting optically pumped semiconductor lasers (OPS-lasers) allow for efficient intra-cavity harmonic-generation, OPS-lasers employ an OPS chip comprising a multilayer (multiple quantum-well) gain-structure backed by a mirror with the mirror being one minor of a laser-resonator. The maximum output power available is primarily based on thermal-management limitations of one OPS chip. Power scaling beyond that limit can be effected by employing multiple OPS-chips in a resonator, or combining the output of a multiple resonators by spectral (wavelength) combination, polarization combination, or some combination thereof.
A similar approach is possible using solid-state thin-disk lasers with intra-cavity frequency conversion. Thermal management is relatively simple in thin-disk lasers, however these lasers typically require (for high power operation) complex resonators configured for multiple incidences of laser radiation or pump radiation in a round trip in the resonator.
Another approach is based on frequency-conversion of an output of a single transverse and axial mode (TEM00), polarization-maintaining near infra-red (NIR) fiber-laser (fiber MOPA) coupled to a resonant field-enhancement resonator. This fiber-laser approach has benefits of cost-competitiveness and energy-efficient power scaling of the NIR power. However, for harmonic-generation, spectrally-narrow radiation with a bandwidth between about 0.5 nm and about 1.0 nm is required, assuming a standard critical phase-matching scheme.
Attempts to narrow the spectral line-width of the signal in fiber-amplifiers using single-frequency oscillators face a buildup of stimulated Brillouin scattering (SBS). SBS, in turn can produce pulses which can cause optical damage to fiber components. By way of example, for a ytterbium-doped (Yb-doped) fiber having a core-diameter of 25 micrometers (μm) and a length of 3 meters (m), the maximum power limited by SBS is between about 150 W and about 250 W. Causing a temperature gradient of temperature along the fiber it is possible to somewhat increase the SBS threshold. Nevertheless, any back reflection of radiation into an amplifier fiber can significantly reduce SBS threshold for the single frequency radiation.
In view of the foregoing it is evident that while the fiber-laser (fiber-MOPA) approach has attractive advantages in cost and energy efficiency, the requirement for single-mode, narrow-band operation imposes an eventual limit in power-scaling. There is a need for a resonantly-enhanced, externally frequency-converted fiber-MOPA architecture that does not require single-mode operation.